Thin film deposition techniques are widely used in the manufacturing of microfeatures to form a coating on a workpiece that closely conforms to the surface topography. In the context of microelectronic components, for example, the size of the individual components in the devices on a wafer is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is chemical vapor deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a solid thin film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction. Such CVD processes are routinely employed in many stages of microelectronic component manufacture, for example.
Thin layers of titanium nitride have utility in a number of applications. For example, the electrical conductivity of TiN makes it a useful component of microelectronic capacitors used in DRAM cells. (See, e.g., U.S. Pat. No. 6,211,033, the entirety of which is incorporated herein by reference, which suggests depositing titanium nitride or titanium carbonitride as part of a microelectronic capacitor electrode.) When processing individual wafers in single-wafer CVD reactors, the wafer is heated to the requisite reaction temperature by a heating plate in contact with the wafer.
When the precursors are introduced to the chamber, a secondary deposit of the reaction product may accumulate on the walls of the process chamber as well. This reaction product will build up over time as successive microfeature workpieces are treated. This build-up can be uneven and can be subject to thermal stresses caused by the variations in temperature encountered when heating successive wafers to the requisite deposition temperature. Over time, the coating can degrade and begin to spall or shed particles that may render the finished workpieces commercially unsalable. In so-called “cold-wall” reactors, the walls of the reactor are cooler than the wafer and less of the reaction product may be deposited on the walls. Cold-wall reactors also reduce the thermal stresses on the secondary deposit by reducing the amplitude of the temperature cycle encountered in treating successive workpieces. Yet, even cold-wall reactors start to deposit particles on the workpieces after a time.
To reduce the risk of particle contamination, the interior of the reaction chamber must be cleaned from time to time. For example, a plasma of an etchant gas (e.g., NF3) can be introduced to the chamber to remove the build-up from the chamber walls. Such plasma cleaning processes or thermal cleaning processes (also known in the art) can lead to other particle problems, though. For example, fluorine from NF3 etchant gas can remain on the walls of the reactor and promote shedding of the secondary deposits on the chamber walls. As a result, the walls of these reactors are commonly conditioned after cleaning by applying a layer of the material to be deposited on the workpieces on the chamber wall before any workpieces are deposited.
Single-wafer reactors have limited throughput. To increase throughput, CVD reactors may be adapted to process a plurality of wafers (e.g., 20–250) simultaneously in a batch process. Such batch CVD reactors have proven very cost-effective for many materials. For some other materials, though, batch CVD processes are not commercially viable. For example, attempts to develop batch CVD processes to deposit TiN on microfeature workpieces in commercial production have had little success. One of the problems encountered in batch TiN deposition via CVD is excessive particle generation. Even with a regular cleaning regimen, batch CVD reactors used to deposit TiN shed particles to the extent that product losses and quality control problems often outweigh the commercial advantages otherwise achievable in a batch process. Consequently, applying TiN films using batch CVD has met with very limited commercial acceptance.
Atomic layer deposition (ALD) is a thin film deposition technique gaining prominence in deposition of TiN and other materials on microfeature workpieces. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules A coats the surface of a workpiece W. The layer of A molecules is formed by exposing the workpiece W to a precursor gas containing A molecules, and then purging the chamber with a purge gas to remove excess A molecules. This process can form a monolayer of A molecules on the surface of the workpiece W because the A molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of A molecules is then exposed to another precursor gas containing B molecules. The A molecules react with the B molecules to form an extremely thin layer of solid material C on the workpiece W. Such thin layers are typically less than 1 nm (and usually less than 2 Å) thick and are referred to herein as nanolayers. For example, each cycle may form a layer having a thickness of approximately 0.5–1.0 Å. The chamber is then purged again with a purge gas to remove excess B molecules.
Films deposited via ALD tend to have higher purity and better conformality to microfeature topography than analogous films deposited via CVD. In addition, ALD is often carried out a lower temperature than CVD processes to deposit analogous materials, reducing thermal stresses on material built up on the inner surfaces of the process chamber. As a consequence, some of the difficulties encountered when depositing TiN in a batch CVD process can be ameliorated and batch ALD processing is garnering attention as a commercially viable process for depositing thin films of TiN on microfeature workpieces. Increasing throughput of batch ALD systems would further enhance the commercial benefit of such systems.